Efficient and fast arsenate removal from water by in-situ formed magnesium hydroxide

MgO nanoparticles have good As-adsorption capacity in treating As-contaminated wastewater but suffer from high production cost. In this study, instead of using pre-formed MgO nanoparticles, we found that in-situ formed Mg(OH)2 from MgCl2 and NaOH reaction exhibited super high arsenate (As(V)) removal efficiency. Only 1.5 mmol/L of in-situ formed Mg(OH)2 could remove more than 95% As(V) within 10 min to make the As contaminated water (10 mg-As(V)/L) meet the municipal wastewater treatment standard, whereas MgO nanoparticles failed. The Mg-As sludge has an amorphous crystal structure while no Mg(OH)2 phase could be observed. As(V) existed uniformly within the sludge which was confirmed by elemental mapping. A precipitation-adsorption-coagulation mechanism might exist, which could relieve the restriction of limited surface area of solid MgO adsorbents. This study not only reveals an applicable method for efficient removal of trace level As(V) from water but also implies the huge potential of in-situ formed adsorbents in water treatment.


Batch experiments
All batch experiments were conducted in a glass reactor contained 200 mL of 10 mg/L inorganic As(V) (Na 2 HAsO 4 •7H 2 O, Sigma-Aldrich) solution which was stirred by a magnetic stirrer.As(V) was chosen as the model As species because As(V) is the major component and the most stable form of As in aquatic environments 28 .For nano-MgO treatment, nano-MgO powder was added to each reactor to desired concentrations under stirring (~ 150 rpm).For in-situ formed Mg(OH) 2 treatment, 0.1 mol/L MgCl 2 and NaOH solutions were added successively drop by drop to each reactor to desired concentration.Specifically, in As(V) removal experiment, there were 5 application dosages (0.5, 1.0, 1.5, 2.0, and 2.5 mmol/L) for both nano-MgO and insitu formed Mg(OH) 2 .All glass reactors were sealed and stirred (~ 150 rpm) at 25 °C for 24 h.For adsorption kinetics, 2 mmol/L of material dosage was chosen based on the As(V) removal experiment, and samples were collected at desired time intervals.
The effect of pH on As(V) removal by in-situ formed Mg(OH) 2 was explored with pH values ranged from 10.0 to 11.5.The pH range started at 10.0 because the pH of solution is ~ 10 when Mg 2+ and OH − react at molar ratio of 1:2 to form Mg(OH) 2 .Initial pH was adjusted by 0.1 mol/L NaOH, and the pH at the end of experiment was also tested.The effect of co-existed ions and humic acid (HA) were conducted with co-existed ions (Na + , SO 4  2− , Ca 2+ , PO 4 3− , and CO 3 2− ) of 10 mg/L or HA of 100 mg/L.All experiments were conducted at 25 ± 1 °C, except for the reaction temperature experiment, which was conducted at temperatures ranging from 25 to 40 °C.
There were three replicates for each experiment.At the end or desired time intervals, approximately 4 mL aqueous sample was collected and filtered through a 0.22 μm membrane filter.The concentration of residual As(V) was analyzed using a liquid chromatography-atomic fluorescence spectrometry (LC-AFS) (ELSPE-2, Guangzhou Pulin Sheng Technology Co., Ltd, China).

Characterization
To further explore the underlying mechanism, the precipitates were collected via centrifugation after coagulation, washed with distilled water and dried at 80 °C.Fourier transform infrared (FTIR) spectra of samples were recorded with KBr pellets in the range of 4000-400 cm −1 using a Thermo Scientific Nicolet 6700 spectrometer.The morphologies of adsorbents were observed using a field emission scanning electron microscope (SEM) (S-4800, Hitachi, Japan).The As elemental mapping images were acquired using energy-dispersive X-ray spectroscopy (EDX) module coupled with the SEM under an electron accelerating voltage of 20 kV.The x-ray diffraction patterns (XRD) were acquired using a powder diffractometer with Cu Kα radiation (λ = 1.5418Å) (D2 PHASER, AXS, Germany).The elemental valence and abundance analysis was conducted by using an X-ray photoelectron Spectrometry (XPS).

Performance for arsenic removal
The adsorption performance of nano-MgO and in-situ formed Mg(OH) 2 were compared by adding them into a simulated As(V) contaminated wastewater and analyzing the As(V) removal efficiencies.The results indicate that in-situ formed Mg(OH) 2 and nano-MgO realized similar removal efficiencies while in-situ formed Mg(OH) 2 realized slightly higher As(V) removal efficiency (Fig. 1).As the concentration of nano-MgO increased from 0.5 to 1.5 mmol/L, As(V) removal efficiency increased from 29.7% to 91.0%, while that of using in-situ formed Mg(OH) 2 increased from 34.2% to 95.7% at the same application levels.In other words, to obtain an As concentration less than 0.5 ppm, which is the municipal wastewater treatment standard in China (GB 8978-1996), more than 2.0 mmol/L of nano-MgO will be needed, but only 1.5 mmol/L of in-situ formed Mg(OH) 2 are needed.Nano-MgO has been proved to possess high adsorption capacity for both organic and inorganic As 10,23 .Thus, these results indicate that in-situ formed Mg(OH) 2 from cheap MgCl 2 and NaOH has similar or even better As(V)-removal capability than pre-formed nano-MgO with a much higher price.
The adsorption kinetics of As(V) on nano-MgO and in-situ formed Mg(OH) 2 were studied, as illustrated in Fig. 2. A removal efficiency growth and equilibrium stages were observed for both chemicals, especially for www.nature.com/scientificreports/in-situ formed Mg(OH) 2 .Specifically, for in-situ formed Mg(OH) 2 , the growth stage was extremely fast and finished within only 10 min, and nearly 99% of As(V) in water was adsorbed.This may be attributed to the abundant active binding sites of in-situ formed Mg(OH) 2 .Because As(V) could be efficiently incorporated into both the inner and outer surfaces of in-situ formed Mg(OH) 2 flocs during the progress of flocs growth.Similar phenomenon was observed in adsorption of As(III) by in-situ formed Ti(OH) 4 24 .On the other hand, the As(V) removal efficiency of nano-MgO, in contrast to that of in-situ formed Mg(OH) 2 , increased very slowly in the growth stage.It took as long as ~ 12 h to reach As(V) adsorption equilibrium.A possible reason might be that As(V) is removed during the phase transition of MgO into Mg(OH) 2 , which is a heterogeneous reaction between MgO and water, and will take longer time than a homogenous reaction between ions in water 23 .In addition, for the in-situ formed Mg(OH) 2 treatment, the formed flocs settled down to the bottom very quickly within 30 min.This will facilitate efficient slurry separation via traditional gravity settling.Based on the above results, 10 min was chosen as the reaction time for the following experiments considering operational convenience and As(V)-removal efficiency.It should be noted that in-situ formed Mg(OH) 2 didn't realized higher As(V) removal efficiency but only accelerated the removal process.Thus, this process can't be explained by a co-precipitation mechanism, which is process-controlled.

Influence factors on arsenic removal by in-situ formed Mg(OH) 2
Usually, pH was a key factor during adsorption of As(V).On the one hand, pH value affects As species in water according to their dissociation constants, which will influence the adsorption capacities of adsorbents 29 .On the other hand, pH value affects the formation of Mg(OH) 2 which needs a slight basic pH condition 24 .In this study, As(V) removal by in-situ formed Mg(OH) 2 was highly pH dependent and basic pH conditions could facilitate  the As removal process (Figure S1).Differently, temperature had little influence on the As removal efficiency from 25 to 40 ℃, possibly due to the high reactivity of Mg 2+ with OH − (Figure S2).
Many ions co-exist with As in wastewater, such as Na + , Ca 2+ , and PO 4 3− .Moreover, HA, a common natural organic matter, was frequently reported to affect As removal [29][30][31][32] .Therefore, it is imperative to consider the presence of coexisting cations, anions, and HA while evaluating As(V) sorption capacity of in-situ formed Mg(OH) 2 .
Here, two kinds of cation, Na + and Ca 2+ , three kinds of anion, SO 4 2− , CO 3 2− , and PO 4 3− , and HA were chosen to investigate the effect of co-existed ions and HA on As(V) removal by in-situ formed Mg(OH) 2 .As illustrated in Fig. 3, Na + , Ca 2+ , CO 3 2− , and SO 4 2− had slight or negligible influence on As(V) removal even at a concentration up to 100 mg/L.
Humic acid showed a slight negative effect on As(V) removal at high concentration of 100 mg/L, but no negative effect was observed at low concentration of 10 mg/L.The resistance to HA is an advantage of using in-situ formed Mg(OH) 2 .It was reported that the As(V) removal efficiency of an iron-cerium bimetal oxide material significantly decreased from 96.19% to 56.12% at the presence of only 10 mg/L HA 29 .Preformed sorbents are ease to encounter HA contamination on the material-water interface and lose their advanced adsorption property 33 .In this study, in-situ formed Mg(OH) 2 has renewed material-water interface along with the in-situ formation of Mg(OH) 2 so that the adsorption property could be retained 25,27 .
In contrast, PO 4 3− was found to inhibit As(V) removal significantly with decreased efficiencies by 28.3% and 58.4% when encountering the interference of 10 and 100 mg/L of PO 4 3− , respectively.This is consistent with existing reports 1,29 .It is known than P and As are congeners in the periodic table and have similar chemical properties.PO 4  3− acts as a competitive ion of AsO 4 3− during As(V) removal using in-situ formed Mg(OH) 2 34 .

Properties of the formed Mg-As sludge by in-situ formed Mg(OH) 2
To explore the mechanism of the superior As(V) removal performance of in-situ formed Mg(OH) 2 , a series of characterizations were conducted on in-situ formed Mg(OH) 2 flocs after As(V) adsorption.Firstly, the crystal structure of the in-situ formed Mg(OH) 2 was characterized using XRD (Fig. 4).Obviously, there was no crystalline MgO or Mg(OH) 2 in the in-situ formed Mg(OH) 2 flocs compared to the standard XRD patterns of crystalline MgO (periclase) and Mg(OH) 2 (brucite).Actually, no apparent diffraction peaks was observed in the flocs, indicating that the in-situ formed Mg(OH) 2 existed as an amorphous form.Amorphous metal hydroxides were frequently reported with bigger specific surface areas, more surface hydroxy groups, and thus better adsorption performance 35 .Moreover, no apparent diffraction peaks were found for As minerals, indicating that As possibly existed as adsorbed ions.These results are consistent with existing reports on As coagulation by in-situ formed Ti(OH) 4 and Fe(OH) 3 24,25 .Micro-morphological results showed that the in-situ formed Mg(OH) 2 -As(V) sludge had a smooth bulk outlook (Fig. 5a).However, large numbers of wrinkles was seen on the surface and side at higher magnification (Fig. 5, b-d), indicating that in-situ formed Mg(OH) 2 possessed a loose structure, which is consistent with its XRD pattern.Notably, the loose amorphous structure of in-situ formed Mg(OH) 2 is completely different from the nanosheet-like Mg(OH) 2 formed from nano-MgO hydrolysis 23 .This may be one of the reasons why in-situ formed Mg(OH) 2 has superior As(V) removal performance, as an amorphous structure usually indicates a bigger specific surface area and more As binding sites.The elemental mapping picture indicates that the removed As(V) element distributed uniformly within the Mg-As(V) sludge (Fig. 6).Thus, As(V) was highly possible encapsulated within the precipitate, rather than adsorbed on the particle surface.It is a common sense that adsorption occurs on sorbent-water interface and is partially or completely reversible.The encapsulation might be the main reason about the high and fast As removal efficiency, which could transform reversible superficial adsorption into irreversible encapsulated co-precipitation and thus alleviate the desorption or second release of adsorbed As(V) back into water phase.
FTIR analysis was used to identify the main functional groups of in-situ formed Mg(OH) 2 after As(V) adsorption.As shown in Figure S3, compared to MgO and Mg(OH) 2 without As(V) exposure, there was an absorption peak appeared at 841 cm −1 in in-situ formed Mg(OH) 2 after reacting with As(V).This can be attributed to the  3− (45.01 eV) (Fig. 7b).The comparable peak intensity indicates high As content in the formed Mg-As sludge.EDX reveals a very low Mg/As molar ratio of ~ 6.3 (Fig. 8).These result indicates a very high atomic utilization efficiency of Mg 2+ during the As(V) removal process, which could hardly be achieved by using MgO nanoparticles.As(V) was quite possibly encapsulated uniformly within amorphous Mg(OH) 2 via coordination rather than forming magnesium arsenate crystal.Similarly, no magnesium arsenate was observed in As(V) removal by preformed commercial Mg(OH) 2 21 .The high Mg atomic utilization efficiency implies a low cost and highly reduced mass amount of Mg-As sludge.Therefore, the post-treatment of the hazardous As-sludge will be easier.

Mechanism of As removal using in-situ formed Mg(OH) 2
Based on the above results, a simplified mechanism is proposed and shown in Fig. 9.During the successive addition of MgCl 2 and NaOH solutions to As(V) contaminated wastewater, amorphous Mg(OH) 2 tiny nuclei (Mg(OH) 2 (in-situ)) are formed with high affinity surface hydroxyl groups and big surface area.Simultaneously, As(V) is adsorbed onto the Mg(OH) 2 surface.Then, the nuclei collide with each other and form bigger flocs with As(V) encapsulated within the flocs.Ultimately, Mg-As(V) composite flocs settle down to bottom and As(V) is removed from water.As the Mg(OH) 2 sorbent is in-situ formed within the As(V) contaminated water, tiny Mg(OH) 2 nuclei are formed with renewed Mg(OH) 2 -water interface from basic Mg 2+ and OH − ions with very abundant adsorption active sites.Meanwhile, the adsorbed As(V) hinders the further growth of Mg(OH) 2 nuclei.Thus, amorphous Mg-As sludge is formed instead of Mg(OH) 2 nanocrystals.

Conclusions
Efficient As(V) removal from water is realized by using in-situ formed Mg(OH) 2 , which is obtained by simply adding cheap MgCl 2 and NaOH reagents.Notably, the removal process is very quick and can be performed within several minutes while traditional methods using MgO nanoparticles need several hours.This process is not only timely efficient but also cost-effective than using MgO nanoparticles as sorbent.Further, the amount of the resulted Mg-As sludge can be reduced significantly and thus adapt better to the demand of mass reduction on hazardous waste disposal.The mechanism still needs further study and a possible explanation is that in-situ formed Mg(OH) 2 has more accessible and high affinity surface hydroxyl groups because of the precipitation reaction starting from basic Mg 2+ and OH − ions.As(V) is incorporated into the inner surface of in-situ formed Mg(OH) 2 agglomerates, hinders its crystallization process, and finally forms an amorphous Mg-As precipitate.Although the feasibility in treating real wastewater in large scale is still unclear, considering the very fast, efficient As(V) removal process and low cost of MgCl 2 and NaOH reagents, this As removal technology based on in-situ formed Mg(OH) 2 has great potential for scale application and will imply a bunch of other in-situ formed nanomaterials for water treatment. https://doi.org/10.1038/s41598-024-72258-6

Fig. 9 .
Fig. 9.The possible mechanism for the enhanced As(V) removal from water by Mg(OH) 2 -in situ.